A vacuum brake power booster of this general type is disclosed in European patent application 0 478 396 A1. The electromagnet in this prior-art vacuum brake power booster which actuates the third sealing seat guided in the control housing is associated with the control housing or, rather, is immovably arranged in a recess thereof facing the vacuum chamber. On independent actuation of the vacuum brake power booster by the electromagnet, relative movement occurs between the control housing or, respectively, a third sealing seat and the valve piston. This means lost travel must be additionally overcome by the electromagnet which already must generate a high amount of actuating force at the beginning of the independent actuation in order to open the control valve in opposition to the forces caused at its valve member.
Also, the problems arising with the assembly of a master brake cylinder which projects into the booster housing, are considered unfavourable and can only be overcome by modifying the assembly dimensions of the brake power booster.
Therefore, the object of the present invention is to propose arrangements which considerable reduce the independent actuating force to be generated by the electromagnet.
This object is achieved by the present invention in that the electromagnet, rigidly coupled to the valve piston, is arranged so as to be slidable in the control housing so that the third sealing seat is synchronously movable with the valve piston.
To realize the inventive idea, in an advantageous improvement upon the present invention, wherein a push rod transmitting the output force of the vacuum brake power booster takes support on the control housing by the intermediary of a rubber-elastic reaction disc, the electromagnet is arranged in a preferably bowl-shaped axial extension of the valve piston, the extension being closed by a closure member which, additionally, permits transmitting the input force introduced at the actuating rod to the reaction disc. This design is compact and does not require an increased axial overall length and which permits to expediently integrate a master brake cylinder connected downstream of the vacuum brake power booster in the booster housing.
In order to obtain a favourable course of the lines of flux generated by the electromagnet, according to another favourable improvement upon the subject matter of this invention, the armature of the electromagnet is partly guided in the closure member, the force transmission between the third sealing seat and the armature taking place by means of a cylindrical pin to which a force-transmitting plate carrying the third sealing seat is secured. These provisions permit to reduce magnetic losses which are caused by stray fluxes or radial forces, respectively.
In a weight-optimized design variant of the subject matter of the present invention, the third sealing seat is provided on a ring which is sealedly guided in the control housing and is connected to the force-transmitting plate by means of at least two retaining arms, preferably opposed in radial direction. Preferably, the force-transmitting plate has a rectangular configuration and can be arranged in a radial groove in the valve piston.
Further, it is preferable that the third sealing seat is interposed radially between the two sealing seats.
Proper functioning of the brake power booster according to the present invention, in particular upon return movement of the movable wall, is ensured in another favourable embodiment in that the third sealing seat is arranged in an axially offset manner with respect to the second sealing seat. Reliable guidance of the armature is provided by guiding the pin in the closure member and in a guide portion arranged in the extension of the valve piston.
Another advantage of the present invention is especially suited for application on the so-called lost-travel minimized vacuum brake power boosters, the valve pistons of which abut on stops on the booster housing via transverse members which are arranged in the control housing with axial play. To ensure that the release behaviour of such a vacuum brake-power booster in the pressure decrease mode corresponds largely to that of a device which is not actuatable by independent force, according to the present invention, the third sealing seat, in the release position, is arranged at a distance from the valve member which is larger than, or equal to, the distance between the transverse member and a stop surface provided in the control housing and allowing to entrain the transverse member on actuation. A spring serves to reset the third sealing seat to its initial position after independent actuation has been effected, which is interposed between the armature and the guide portion and biases the armature in opposition to the actuating direction of the electromagnet.
In order to initiate a controlled braking operation or a desired delay, respectively, in another favourable design variant of the present invention, the third sealing seat is operable only to the effect of separating the chambers. A valve operable independently of the electromagnet is provided which allows the working chamber to be ventilated irrespective of the control valve. These provisions permit a clear separation of two functions which, otherwise, have to be performed merely by the third sealing seat (no pulsed control being required for realizing a mode in which the pressure is maintained constant). The valve designed as a pneumatic solenoid valve (which is closed when deenergized) is arranged at the housing half confining the working chamber, thereby accomplishing an additionally enhanced reliability in function of the vacuum brake power booster according to the present invention, in particular when power failure occurs.
Improving or, respectively, optimizing the relationship between the resetting force acting upon the second sealing seat and the reaction force of the vacuum brake power booster according to the present invention is achieved in another favourable embodiment of this invention by the valve member in the control housing confining a pneumatic chamber, to which the pneumatic pressure prevailing in the working chamber is applicable.
A particularly short flow path upon ventilation and evacuation of the pneumatic chamber or, respectively, a rapid change in pressure in the pneumatic chamber is achieved in another embodiment of the subject matter of the present invention by the valve member being provided with at least one passage in the area of its sealing surface, which forms a connection between an annular chamber confined by the sealing seats and connected with the working chamber, and the pneumatic chamber.
Finally, another favourable embodiment of the present invention is characterized by the valve member being provided with a radially inwardly disposed sealing lip in the area of its sealing surface which lip, together with a retaining ring holding the valve member in the control housing, confines the pneumatic chamber. Effective sealing of the pneumatic pressure compensating chamber is ensured by this provision.
To permit a specific, adjusted introduction of the independent brake force, which is not initiated by the driver but is to be generated by the electromagnet, in another embodiment of the subject matter of the invention, a means is provided which permits detection of the actual position of the valve member. The adjusted introduction of the independent brake force accomplished by this measure is necessary in all possible applications in which not only the maximum independent braking pressure, not initiated by the driver, is desired, but, for example, a precisely adjusted period of preservation of the aimed-at actuating condition.
Preferably, the mentioned means permits direct sensing of the actuating travel of the valve member. In the direct sensing, the actuating travel of the valve member is sensed by means of a Hall effect sensor. Two embodiments are possible.
In the first embodiment, the Hall effect sensor is accommodated in the control housing, preferably in the area of the first sealing seat, and cooperates with a permanent magnet which is integrated in the valve member. In the second embodiment, the Hall effect sensor is provided in the valve member, while the permanent magnet is arranged in the control housing, preferably in the area of the first sealing seat.
With respect to the modifications which, for the direct sensing, are necessary to be made to the valve member manufactured in large-scale series production, it is further suggested to provide a means which permits an indirect sensing of the actuating travel of the valve member, preferably, sensing of the actuating travel of the armature of the electromagnet. For this purpose, the actuating travel of a ring carrying the third sealing seat is defined, which ring cooperates with a force-transmitting plate coupled to the armature. According to the present invention, a permanent magnet is arranged in the force-transmitting plate to this end which cooperates with a Hall effect sensor integrated in the valve piston. In this type of sensing, there is no proportional relation to the actuating travel of the valve member until the third sealing seat abuts on the valve member so that this area must be taken into account in analyzing the output signal of the Hall effect sensor, for example, by way of a microcontroller.
Another principal possibility of indirect sensing is to provide a means which permits sensing the density of the magnetic flux in the working air gap of the electromagnet, while the current supplied to the electromagnet is measured simultaneously. Preferably, the working air gap of the electromagnet is confined by the armature, on the one hand, and by a cylindrical guide portion designed within the valve piston, on the other hand.
A favourable improvement upon the last-mentioned embodiment is that an analogously operating Hall effect sensor is arranged in the guide portion on its side facing the armature. This provision permits to continuously sense the flux density of the electromagnet.
In order to prevent the decrease of the end surface of the guide portion which is required to generate the electromagnetic force, according to another embodiment of the subject matter of the invention, the Hall effect sensor is arranged in the return path of the electromagnet, preferably, and is preferably integrated in the wall of a cylindrical extension of the valve piston which accommodates the electromagnet.
To ensure that the independent actuation of the brake power booster according to the present invention is in fact completed, in particular after an auxiliary-force-assisted braking operation has already been performed, in another favourable embodiment of the subject matter of the present invention an electrical switching means is provided, the control signals of which influence the current supply of the electromagnet, more particularly permit its disconnection. Preferably, the switching means is a microswitch arranged at the valve piston as well as an actuating element arranged at or in the control housing.
The switching travel of the mentioned microswitch is extended in an advantageous improvement upon the present invention in that the switching means is operable by a relative movement between the valve piston and the control housing. It is particularly favourable that the actuating element is arranged so as to be movable relative to the control housing and resiliently biassed in opposition to the actuating direction. The actuating element abuts on a stop on the booster housing under the preload of a compression spring in such a manner that an axial clearance forms at its end close to the microswitch between it and the control housing.
An effective tolerance compensation at the actuating element is achieved according to another favourable embodiment of the present invention in that the actuating element is formed of two resiliently biassed parts which are telescopically guided in one another, in particular an actuating tappet that is sealedly guided in the control housing and cooperates with the stop, and a sleeve which is anchored at the actuating tappet in a resiliently biassed manner, which is arranged coaxially to the actuating tappet and cooperates with the microswitch. This provision reliably prevents any damage to the microswitch.
To ensure a reliable disconnection of the electromagnet, in particular after a braking operation assisted by independent force has been completed, in another embodiment of the subject matter of the present invention, the valve piston is in axial abutment with a reaction disc, transmitting the output force of the vacuum brake power booster, by means of a transmission disc which is mounted in the control housing so as to be axially movable within limits, the movement of which, in "opposition"to the actuating direction of the vacuum brake power booster, is limited by a stop (annular surface) provided in the control housing and allowing further movement of the valve piston in opposition to the actuating direction.
Finally, the assembly of the vacuum brake power booster according to the present invention is facilitated in that the control housing is of bipartite design and includes a front part, which is positively connected with a guide portion accommodating the control valve, and which accommodates the force output member (push rod), the reaction disc and the transmission disc.